1. Field of the Invention
The present invention relates to an inverter device for driving an electric motor, which drives an electric motor provided in a construction machine or the like on the basis of a PWM control scheme.
2. Description of the Related Art
In a conventional inverter device for driving an electric motor, a carrier frequency that determines a pulse width modulation period for pulse width modulation (PWM) control is generally set to a value that enables control stability at the maximum rotational speed of the electric motor to be secured, being constant regardless of changes in the rotational speed of the electric motor (this technique is hereinafter referred to as a first known technique).
On the other hand, there is also known a technique which involves changing the carrier frequency in accordance with a certain element. For example, Japanese Patent Application Laid-open No. 2009-291019 discloses changing the carrier frequency in accordance with a voltage instruction value, or output torque, which technique is hereinafter referred to as a second known technique. According to the second known technique, a higher carrier frequency is set for a region of higher output torques while a lower carrier frequency is set for a region of lower output torques. This enables a reduction both in current ripple and in switching loss to be expected.
According to the first known technique, the carrier frequency needed in terms of control stability is high in a speed region where the electric motor rotates at high speed and decreases with the decrease in the rotational speed of the electric motor. Thus, the first known technique involves higher carrier frequency than necessary in a low speed region, thereby increasing the switching loss with decreasing rotational speed of the electric motor and reducing inverter efficiency.
On the other hand, the second known technique takes no account into the electric-motor rotational speed in connection with changes in carrier frequency. Hence, in the case where a large torque is required in the low speed region, there cannot be obtained a high inverter efficiency, and in the case of a small torque is required in a high speed region, there cannot be obtained an excellent control stability.